Is Theory Necessary for Chemistry?

What do you imagine when you think of chemistry? You may believe that chemistry deals with change and diversity of substances and requires memorization of the periodic table, chemical formulas and reaction formulas. Chemistry in the scope of junior high and high school education is generally thought of in terms of arranging various properties of substances systematically. In fact, that image is right to a certain extent. However, modern chemistry cannot be explained only by that. When the Schr?dinger equation, which is an equation of motion representing the movement of an atomic nucleus and electrons, was found in the early twentieth century, chemistry changed significantly to a study that holds the possibility to predict all properties of all substances by solving this equation. Chemistry for understanding and predicting various properties of substances based on the Schr?dinger equation is called quantum chemistry. Dr. Linus Pauling, who received the Nobel Prize in Chemistry in 1954 and is also known as the father of modern chemistry, developed the concept of covalent bonds and hybrid orbitals and greatly contributed to the establishment of the modern chemical theory based on quantum chemistry. Many Japanese chemists have made extraordinary contributions to the development of quantum chemistry. In 1981, Dr. Kenichi Fukui of Kyoto University became the first Japanese to receive the Nobel Prize in Chemistry for research on establishing quantum chemical theory for chemical reactions. I entered the Department of Chemistry II in the Faculty of Science at Tohoku University in 1982 when people were still excited by Dr. Fukui’s award of the Nobel Prize in Chemistry. Soon after entering university, I was invited to join the Quantum Chemistry Study Circle and started studying quantum chemistry with extreme difficulty. I think this event affected the direction of my research later. When I became a senior and selected a laboratory, I decided to conduct research on quantum chemistry without hesitation. After graduating from the master’s program, I felt a stronger compulsion to directly verify prediction of theoretical chemistry through my own experiment. I became involved in a field of research that conducted experiments using laser photochemistry in which a theoretical prediction is most directly reflected. These years I am fascinated with the prediction of thermodynamic nature generated from polymer motion in a solution, which is a complex system, and with solving the equation of motion using a computer.

 

Laser Light and Photochemistry

Today, there is a large number of equipment containing laser (Light Amplification by Stimulated Emission of Radiation) devices including a laser pointer and a laser printer. However, laser light equipment has a relatively short history. Although the first successful laser oscillation was made in 1960, it was in the 1980s that laser equipment became practical at a laboratory level, and after the 1990s that its utilization at a commercial level started. Furthermore, ultrashort pulse lasers that generate ultrashort time pulse light in femtoseconds (10^-15 seconds) currently continue to evolve. The straight advance property, monochromatic property and coherence, which characterize laser light, are determined by the quantum chemical properties of the laser oscillation medium. These characteristics of laser light enable the measurement of quantum chemical properties of a substance that cannot be measured with a normal light source such as an incandescent lamp. I would like to elaborate on the necessity and usefulness of quantum chemical theory calculation, using my research "measurement of resonance quantum beat of π-electron vibration with use of a phase-lock laser pulse pair" as one example among my research projects using laser light.

 

Resonance Quantum Beat of π-electron Vibration

Figure 1 (a) shows an anthracene dimer used for an experiment of π-electron vibration. Anthracene belongs to a molecular system called polycyclic aromatic, which is made by condensing the hexagonal carbon atom framework. Graphene, which is recently gaining attention as a nanomaterial, also belongs to a polycyclic aromatic. An aromatic has a characteristic of having an electron that can move freely in the molecule, called π-electron. Due to this characteristic, the molecular system is expected to be used in various applications including conductive materials. By irradiating an ultrashort pulse laser on an anthracene, the vibrational movement of a π-electron can be generated (Note 1). The π-electron vibrations generated in each of two anthracenes resonate with each other.

In this time, beat vibration (quantum beat) occurs because the frequencies of the π-electron vibrations are slightly different from each other (Note 2). Figure 1 (b) shows quantum beat signals measured by a device that we developed. This phenomenon can be compared to the more familiar resonance of two tuning blocks having frequencies slightly different from each other (Figure 1 (c)). However, the experiment of the anthracene dimer included an element which was not connected to the resonance of the tuning blocks. That element was the possibility that the experiment measured nuclear vibration forming the molecular frame. Does the measured result indicate the π-electron quantum beat? Or nuclear vibration? To reach a conclusion, a simulation was conducted by solving the Schr?dinger equation. Figure 2 shows the result of the simulation. As shown in the figure, two beat curves of π-electron vibration (Note 3) cross one another, and no crossing occurs for the curves of nuclear vibration. For the signal observed in the experiment, the two curves clearly cross, and therefore it can be concluded that the vibration measured was π-electron resonance. I think it is important for a front-line chemistry experiment to be supported by theoretical calculation in all cases not just limited to this example.

 

Notes

(1) Strictly speaking, vibration between the ground state and the excited state of the wave function of the π-electron.

(2) A phenomenon called exciton splitting.

(3) One beat curve of the same phase as the laser pulse and the other curve of the phase opposite to the laser pulse (different by 180 degrees)